Work will continue along two lines, using developing tissues: 1) We found earlier that thyroxine stimulates transport of alpha-aminoisobutyric acid (AIB) into brain slices from rats 5 to 24 days old, the age at which rat brain develops most rapidly. Amino acid transport systems will be analyzed in brain slices from normal, thyroxine-treated, and propylthiouracil-treated animals to determine specific changes produced by thyroxine. Procedures will include: altering extracellular (Na ion), pH, temperature; inhibition by other amino acids, and by competitive inhibitors; transport of other neutral amino acids. These factors can distinguish the major transport systems for neutral amino acids ('A', 'L', 'ASC'). Brain slices from thyroxine-treated rats under 5 days old will also be studied, to determine how early thyroxine can alter transport. 2) Earlier we showed that AIB transport in diaphragms from 1-day rats does not respond to insulin added in vitro. Responsiveness is small at age 3 days, and increases to a doubling in 30-day-old diaphragms. Other transport characteristics indicate presence, at 1 day, of the 'A' system, which is sensitive to insulin in older diaphragms. Future work will determine in more detail the transport systems in 1-day diaphragm, with special emphasis directed toward explaining the lack of insulin-sensitivity.